1. Field of the Invention
This invention relates to a compact, economical and efficient humidifier system for providing a supply of air at or near 100 percent relative humidity and a predetermined temperature to a medical (e.g. critical care) patient. The humidifier system is of the type which is adapted to be interfaced with an artificial respirator (i.e. ventilator) such as that commonly found in an intensive care unit of a medical facility.
2. Prior Art
Medical patients that are confined to an intensive care unit of a medical facility are commonly provided with a supply of breatheable air from an artificial respirator (i.e. ventilator) source. However, it is medically preferable to control the relative humidity and temperature of such air being delivered to the respiratory passages of the patient. More particularly, apparatus is desired which is capable of delivering, to a patient, air that is characterized by substantially 100 percent relative humidity and a temperature of 35.degree. C. That is, breatheable air that is highly saturated with water vapor will minimize the drying out of the mucous membranes located in the patient's respiratory tract so as to lessen the chance of injury to delicate tissues and thereby reduce the possibility of an obstruction in the patient's breathing passage as a consequence of insufficient mucous flow. Moreover, it is advantageous to heat the patient's supply of breatheable air to a temperature which is at or near body temperature (e.g. 35.degree. C.-37.degree. C.), because vapor saturated air at body temperature contains more water vapor than vapor saturated air at room temperature (e.g. 20.degree. C.-24.degree. C.).
Accordingly, various humidifier devices are available to be interfaced with a ventilator for heating and humidifying a supply of air for delivery to a patient. However, such devices are typically characterized by one or more of the following shortcomings. By way of example, certain conventional devices, known as pot humidifiers, require a large reservoir supply of water (e.g. as much as 300 to 600 cc) to be heated for the purpose of generating water vapor. Due to the large thermal mass created by such a large reservoir supply, pot humidifiers are generally cumbersome and energy inefficient and usually require a relatively lengthy period of time from start-up before the reservoir water supply can be heated to a temperature suitable for producing vapor.
Moreover, in some pot humidifiers the characteristic large mass of water is overheated in order to produce vapor, with the heat of vaporization being used to compensate for heat losses in the delivery tubing which supplies humidified air to the patient. However, resulting condensate which is formed within the tubing must be drained at regular intervals. Such a humidifier system is inherently heavy and slow reacting. In addition, the system must be physically spaced from the patient to avoid excessive temperature fluctuations within the humidified air delivered to the patient. Humidifier systems which incorporate means to heat the tubing to overcome the foregoing are typically expensive and/or undesirably complex.
Other conventional humidifier devices, known as wick humidifiers, utilize an absorbent wick for the purpose of reducing the size of the reservoir water supply. However, and as will be known to those skilled in the art, such wicks have a relatively short lifespan and, therefor, require frequent replacement. In some cases, the wick is formed from blotting paper which is positioned in contact with a source of heat so as to permit the wick to rapidly reach operating temperature. However, such blotting paper is typically not reusable and both the cost of maintenance and time for reconditioning the wick humidifier are correspondingly increased.
Still other conventional humidifier devices lack the ability to precisely control the temperature of the air being delivered to the patient. Such devices do not include means to accurately sense the temperature of the air at both the humidifier outlet and patient ends of an air delivery (e.g. tubing) system. Thus, temperature variations which are frequently produced by changes in the air flow between the humidifier and the patient are often overlooked.
Reference may be made to one or more of the following United States patents for examples of a conventional humidifier system:
U.S. Pat. No. 3,282,266; Nov. 1, 1966 PA0 U.S. Pat. No. 3,434,471; Mar. 25, 1969 PA0 U.S. Pat. No. 3,638,926; Feb. 1, 1972 PA0 U.S. Pat. No. 3,659,604; May 2, 1972 PA0 U.S. Pat. No. 4,248,217; Feb. 3, 1981 PA0 U.S. Pat. No. 4,284,878; Aug. 18, 1981 PA0 U.S. Pat. No. 4,305,388; Dec. 15, 1981 PA0 U.S. Pat. No. 4,369,777; Jan. 25, 1983 PA0 U.S. Pat. No. 4,430,994; Feb. 14, 1984
By way of particular example, U.S. Pat. No. 3,434,471 discloses a humidifier for use with a ventilator. The humidifier includes a reservoir with a heat exchanger and heater. Air to be humidified is passed into the heat exchanger, whereupon it is heated. The heated air then passes above the water in the reservoir. Thus, the air becomes saturated with water vapor and is passed through a second heat exchanger, whereupon it is cooled to cause some moisture to "rain out" and thereby assure 100 percent relative humidity at the temperature at which the air exits the humidifier. The temperature of the air at the patient end is sensed by a thermistor which controls the output of the heater.
U.S. Pat. No. 3,638,926 discloses humidification and heating apparatus which comprises a spirally wound resistance-heated plate in thermal contact with a sheet of water absorbent blotting paper. Air from a respirator travels along a spiral passage formed by the plate, the water surface at a reservoir, and a lid to produce a substantially saturated vapor at a desired temperature.
U.S Pat. Nos. 4,248,217 and 4,305,388 generally relate to devices for controlling the heating of inspiratory gas as a function of the expiratory gas temperatures. Thermistors are used in each of the inspiratory and expiratory flow paths for sensing and controlling the heating of the gas.